Technical Field:
The technical field relates generally to turbo-charged compression ignition engines having exhaust gas recirculation and, more particularly, to methods of controlling a variable geometry exhaust turbine in the turbo-charger sub-system.
Description of the Technical Field:
Exhaust turbines have long been used to recover energy from high pressure exhaust gas produced by most internal combustion engines. One use of the recovered energy is to drive a compressor/supercharger on the intake side of the engine and thereby increase the mass of the air charge delivered to engine combustion chambers. This is referred to as turbo-supercharging, or more commonly just turbo-charging. The use of turbo-charging to increase the mass of air delivered to the engine allows more fuel to be burned per combustion stroke thereby increasing the work done with each combustion stroke. The system is a self reinforcing loop as the engine serves as the pump which drives the exhaust turbine.
Variable geometry turbines (VGT) are used with turbo-charged engines to reduce engine pumping losses. U.S. Pat. No. 6,067,800 describes use of a VGT for improving torque response, fuel economy and emission levels.
Exhaust gas recirculation (EGR) is commonly employed on compression ignition engines to reduce nitrous oxide emissions. An EGR valve connects the exhaust manifold of an engine to its intake manifold. Exhaust gas displaces fresh air in the combustion cylinders and functions as an inert gas in the cylinder reducing cylinder temperature during combustion and thus reducing the formation of nitrous oxides. The concurrent use of EGR and turbo-charging with an engine complicates control over the engine. In a typical engine control system where both turbo-charging and EGR are present, a VGT has been used to control mass airflow (MAF) in the intake manifold and the EGR valve has been used to control emissions.